The invention relates generally to combined-cycle (CC) power generation systems and, particularly, to scheduling the startup of a CC power generation system (also referred to herein as a “plant”).
Plants are typically operated by or for power utilities that generate power which is ultimately dispatched via central wholesale market or regulated power system operator. A power utility may generate power from one or more CC power generation systems and other power generation systems.
The power demand on utilities tend to vary hour-by-hour, day-by-day, season-by-season and year-to-year. The power demands from their customers are forecasted by, for example, grid operators based on historical data regarding power demand and other information such as expected weather, requests for future power by customers and events scheduled to occur that impact the demand of power. The grid operators advise the power generation operators, e.g., utilities of the forecasted power demands. Because the power demand varies, the schedules prepared by power utilities for each plant are often finalized a short time before, e.g., the day before, the power is to be generated. Once the schedule is finalized, the operators of power generation systems determine when to start the CC power generation system (plant) to provide power at the dispatchable load level when the schedule indicates that the demand for the power will occur.
Determining when to start a plant poses a complex and difficult scheduling problem. The startup sequence takes a plant from an off condition to the condition at which the plant produces power at a dispatchable load level. Startup sequences are typically complicated schedules involving various gas turbines, steam turbines, boilers and other systems to generate steam, and electrical generators driven by the gas and steam turbines. When a plant is stopped, the gas turbine(s) are not being provided with fuel and the steam turbine(s) are not being provided with steam. When the plant is stopped, the gas turbine(s) and steam turbine(s) cool from their last operational condition. When the plant is restarted, the duration of, or time to complete, the startup sequence is dependent to a large extent by the temperature of the steam turbine(s) when the startup sequence is initiated.
It would be helpful to an operator of a plant to have a tool to accurately calculate the duration of a startup sequence. The operator typically knows when his plant is committed to produce power at a dispatchable load level. Knowing an accurate startup duration, would enable the operator to start the plant at the latest possible time and using the least amount of fuel so that the dispatchable load is reached just before the plant is committed to produce power.
To determine when to initiate a startup sequence requires the system operator to estimate the length of time required for the sequence. Calculating an accurate, condition-based startup schedule for a CC power generation system can be a laborious and complicated task conventionally performed manually by a system operator, due to the difficulty in accurately forecasting the duration of a startup sequence. Rather than calculating an accurate, condition-based startup schedule, plant operators typically forecast the startup schedule using previously prepared templates of startup periods for a few startup conditions. The prepared startup templates conservatively predict long startup periods to ensure a predicted startup period is never shorter than any of the variations of startup sequences to which the estimated period is applied. Because they are conservative, the prepared templates of startup periods may be applied generically to a broad range of initial conditions, such as the temperature of the steam rotor at the start of a startup sequence. The prepared templates may state the startup period significantly longer than most actual startup periods.
While reusing existing schedule templates expedites the preparation of a new startup schedules for a new operating day or time period, schedule templates often do not yield optimal startup sequences and startup duration for any given day or time period. Moreover, previously prepared schedules may incorporate long margins of time to ensure that the various power generation components are available to suit all of the potential situations to which the schedule may be applied. These long margins result in power generation components becoming available for use, e.g., dispatchable load, up to hours before the components may be needed and unnecessarily burning fuel inefficiently at low load levels. Arriving at a dispatchable load earlier than needed results in monetary losses due to power components being operated while the components wait to be applied to generate needed power and resulting in generated power being sold at sub-optimal price levels.
A difficulty with the prepared estimates of the startup duration is that the plant may reach the dispatchable load level a half hour or more before the plant is scheduled to produce dispatchable power. While the plant generates power before it is scheduled to provide power, the plant consumes fuel, generates excess heat and emissions, may operate at a relatively low efficiency, and the operator may be forced to sell the power at below market prices. A plant operating at dispatchable load without a sufficient demand is undesirable. Increased cyclic duty requirements, higher fuel costs, competitive deregulated energy markets, and stringent environmental regulations create a demand for faster and more predictable startup sequences from CC power generation system operations.
There is a long felt need for methods and systems to easily, quickly and accurately generate schedules and forecasts to startup CC power generation systems. The need exists because the conventional manual approach to selecting one of a few prepared startup schedules results in inefficiencies, such as those due to power generating components, e.g., gas turbines and steam turbines, that reach dispatchable load levels hours before these components are actually needed. Further, the requirements that power plants bid to generate power increases the need for accurate scheduling and forecasting tools to generate startup schedules quickly and that are optimized to reduce the cost of generating power.
Reducing the time required to start a CC power generation system is not the only consideration when starting the system. Power generation system owners manage different startup objectives depending on local environmental regulations, energy dispatch requirements, and current fuel and energy prices. The power generation system operator may need to minimize emissions, fuel costs, or net heat rate. These considerations may affect the timing of the startup operation. Each CC power generation system may have site specific factors that affect the startup schedule. There is a long felt need for startup schedules that accurately predict the startup duration for a variety of startup operating conditions of a CC power generation system.